Technological performance is evolving towards an increasing density of storage, and towards an increasing speed of reading recorded media. High information density and a fast reading speed are particularly concerned with the bandwidth of the transmission channel between the recording medium and the device for retrieving the stored information.
Approaches have been developed with success for communication applications and for magnetic recording media. For example, the coding of information on hard disks often uses the known partial response (PR) codes. These are multilevel codes whose signal spectrum is easily adaptable to the transmission channel for a given running speed. The retrieval device then uses a maximum likelihood (ML) decoding. The various levels of the signal transmitted in analog form are converted into a digital signal by samplings of an analog digital converter (ADC) with several bits. However, these systems which are known as PRML, implement a digital filtering which does not allow the use of a single-bit ADC converter.
To obtain a high density storage on the information medium, the information may be coded using symbols of various lengths on two possible aspects of a surface of the medium. For example, on an optical disk, a presence and an absence of a hole differently reflects a light ray causing binary modulation of a transmission signal. This makes it possible to code a symbol as a hole whose length is representative of this symbol, and to code the following symbol directly by an absence of a hole whose length is likewise representative of this following symbol. It is thus possible to code a succession of symbols on the medium by a succession of holes and an absence of holes without losing room. The symbol length is obtained directly in digital form by counting clock periods on the same aspect of the surface of the medium, then similarly on the same complementary aspect for the following symbol. It would then be beneficial to use a single-bit analog-to-digital converter.
However, the evaluation of symbol lengths is disturbed by numerous phenomena, such as the transfer function of the transmission channel, its sensitivity to noise having a random nature and inter-symbol interference. Interference between symbols, although not generally exceeding two symbols, is particularly acute for symbol lengths on the order of 0.42 μm with a reading beam diameter on the order of 0.74 μm.
The devices of the prior art particularly require careful calibration and give rise to constraints in the design of the analog circuits and in their parameterization.